The present invention generally relates to a switch for a communication network, and more particularly to a cross-connect switch that utilizes a grid of cathodes that generate free space electrons. The free space electrons are accumulated and directed toward a grid of receiving anodes.
Virtually all of the telecommunications backbone of the nation consists of highly specialized fiber optic systems. Although photons are ideally suited for transmission through a solid medium, because they are highly non-reactive both to their medium and to each other, they are ill suited for processing and switching. Purely optical switching has proven difficult since photons cannot be steered without modifying the physical medium through which they travel, for example by reflecting them off of aimable mirrors or by passing them through variable-twist LCD molecules or temperature-sensitive crystals. The process of modifying the physical medium in order to steer a photon beam tends to be slow and unwieldy; few photonic switching technologies are fast enough for packet-by-packet switching, and the ones that (binary, two position micro-mirrors) cannot be scaled to sufficient port counts.
One method of switching photons is MEMS-Based Movable Mirrors Switches. Movable mirrors switches fall into two categories-switches that use infinitely adjustable mirrors (analog MEMS switches), and switches that use two position mirrors (digital MEMS switches). Digital MEMS switches has potentially very low switching latency, but they are not scalable. The number of internal components in a digital MEMS switch increases exponentially as the number of ports increases, making them difficult to scale beyond just a few hundred ports. A 1,000 port digital MEMS switch would require about 240,000 mirrors, and 2,000 ports would simply be unattainable. As a result, all large-scale MEMS switches use analog, infinitely adjustable mirrors, which allow for greater scalability. It has been reported that analog MEMS switches with over 1,000 ports are close to production. However, it will take several years for these switches to scale beyond 4,000 ports. Additionally, these analog MEMS switches have very high switching latency; all existing switches require milliseconds to switch, and this is not likely to decrease in the foreseeable future.
To date, serious questions exist about the longevity and reliability of MEMS switches. For example, the longest-living analog MEMS switch survives on the order of one billion switching cycles. Therefore, if any analog MEMS switch could switch quickly enough to switch packets at commercially acceptable rates, it would barely survive one minute before reaching the end of its operating life. Furthermore, MEMS switches are sensitive to shocks and are fragile. Another disadvantage of the current generation of MEMS switches is that they are bulky. For example, a 1152 micro-mirror port switch produced by Xros is purported to occupy 2xc2xd 7-foot bays. The footprint of MEMS switches is likely to decrease in the future.
Finally, the ability to switch without the use of regenerator lasers and their requisite electronic conversion is widely considered to be the key advantage of photonic switches such as MEMS switches. However, practical lambda-by-lambda switching requires more than passively redirecting lambdas from fiber to fiber. In order to prevent wavelength xe2x80x9ccollisionsxe2x80x9d, it is necessary to change the wavelength of the lambdas as they hop from switch to switch. This requires the use of regenerator lasers. Tunable lasers do not mitigate this problem, since they still require that a given wavelength be reserved from end-to-end of the network. The collision problem can be attacked either by wasting circuits, i.e., by making available many times the number of circuits than are strictly necessary to handle the required bandwidth while avoiding wavelength collisions, or by using regenerating lasers at each switch hop to change the wavelength of the lambdas as needed to avoid collisions. The inevitability of significantly less expensive lasers, and the high cost of circuits given the low port count of today""s switches will heavily weigh the argument in favor of using more lasers rather than creating more circuits.
It is worth noting that all current generation MEMS switches require the use of regenerators even in coarse, fiber-by-fiber switching applications, because of a lack of reflectivity in the mirrors. Several manufacturers purportedly have found ways to increase the reflectivity of their mirrors, e.g., by gold-plating them. However, it is not clear that this will eliminate the need for regenerator lasers, especially in real-world networks that have multiple hops and long-haul links. Overall, it seems very likely that MEMS switches will continue to require regenerator lasers for any real-world lambda-switching application.
Several other photonic switching technologies compete with MEMS switches in optical switching applications. These include the Agilent xe2x80x9cbubblexe2x80x9d switch, LCD switches from several manufacturers, switches that steer light using temperature-sensitive crystals, and others. However, these technologies suffer from a lack of scalability (LCD switches and bubble switches) and high switching latency (all of them). Early claims that LCD switches might be able to switch at nanosecond speeds in the foreseeable future have proven untrue.
Another method of electronic switching is by the use of single-stage crossbars. A crossbar is a semiconductor-based logic device that is used for switching. The main disadvantage of single-stage crossbars is scalability: the number of internal components in a crossbar increases exponentially or nearly exponentially as the number of ports increases. As a result, most existing crossbars have a maximum of 64-ports. New but very complex internal interconnect schemes allow port count to be increased to 512. However, neither type of crossbar is likely to increase in size beyond that in the foreseeable future, since a large increase in the number of internal components is needed to realize an incremental increase in port count.
Crossbars are also limited by the clock speed of their logic gates, which is typically at or below a single GHz. To obtain higher port speeds, multiple slower ports must be combined in order to create a single fast port, which greatly decreases overall port count. For example, with a crossbar that runs at 622 MHz, 66 ports must be combined to create a single OC-768 port. Also, the demultiplexers and multiplexers that separate the bit stream and then recombine it is complex and requires exotic technology, especially for OC-192 bit rates and beyond.
Clos is an interconnection topology that allows smaller crossbars to be combined to form a larger, higher port count switch. Almost all-existing and planned crossbar-based switches are built using the Clos topology. For example, Growth Networks was a switch startup that was developing a 512-port OC-48 Clos-interconnected crossbar switch.
Clos requires a large number of crossbars in order to obtain a given port count-roughly 3.5 times the total port count divided by the number of ports per crossbar. As a result, Clos-interconnected crossbar switches have very large footprints-the Growth Networks switch will require a full 7-foot tall bay for 512 OC-48 ports. Also, all of those crossbars ICs consume a huge amount of power.
Latency (switching speed) is also a problem with Clos switches. Semiconductor Clos switches typically require tens to hundreds of microseconds to establish a connection from an input port to an output port. Moreover, their switching latency is non-deterministic, which means that the amount of time needed to establish a connection is highly unpredictable. In packet-by-packet switching applications, this greatly increases the complexity of the packet forwarding engines and traffic managers that control the switch, since it is difficult for the switch to guarantee FIFO packet behavior. It also introduces unwanted effects into the output packet stream such as jitter. Furthermore, these problems become worse as the switch becomes larger. It is likely that many of the Clos crossbar-based electronic switches that are used within OEO optical cross-connects have so much latency, and such non-deterministic latency, that they would be unsuitable for packet-by-packet switching.
Board to board connector density is also a serious problem with Clos switches. In large Clos switches, nearly four out of every five interconnects are internal to the switch and cannot be used for external, through-switch bandwidth. Therefore, Clos-based switches are limited by the connector density, trace density, and interconnects needed to create all of this internal intra-switch bandwidth. As a result, it has been suggested that Clos switches hit hard limits in terms of board-to-board connector density at 512 ports.
Also, because of their high component count, reliability is an important issue with semiconductor Clos switches. Any switch that consists of a full bay of ICs must support complex failure-recovery and rerouting capabilities. Finally, as with all semiconductor logic-based switches, bit rate per port is limited by the clock rate of their logic gates. The same issues that limit single stage crossbars limit multi-stage crossbars. With a clock rate of 622 MHz, 66 ports would have to be combined to create a single OC-768 port.
On the other hand, electrons are ideally suited for switching. Electrons can be easily steered by electrostatic and electromagnetic fields. However, previous electron switches have steered electrons through digital logic gates on semiconductors. These devices have proved complex and difficult to scale in switching applications, and they are limited by the slow speed at which their solid-state logic gates are capable of switching.
The switch of the current invention steers electrons through freespace rather than through semiconductors, in a manner that is similar to a CRT display. In a CRT display, electrons travel from the electron gun that is at the back of the CRT to an array of phosphors at the front of the CRT. The beam from the electron gun is magnetically steered to selectively illuminate the phosphors. The switch uses an array of electron emitters rather than a single electron gun. Each input port is associated with an electron gun and each output port is associated with an electron detector, which is implemented as a simple conductor. Data is transmitted from an input port to an output port by electrostatically aiming the input port""s electron beam toward the output port""s detector, and then modulating the beam.
Although the switch of the present invention converts photons to electrons and then back to photons, and is an electronic switch, it does not use the slow, bulky semiconductor-based logic devices that the term xe2x80x9celectronicxe2x80x9d has come to imply. In fact, it is not even a digital switch. It simply creates an analog transmission line from the input port to the output port. Moreover, this transmission line is an ideal transmission line, with low impedance (even freespace has some impedance), very fast propagation, zero voltage drop (it can even amplify the signal), and no crosstalk. This transmission line has almost unlimited throughput, and can operate at OC-768 speeds and beyond. The switch uses electrons for precisely what they are best at, and as a result it is better than either photonic or traditional electronic switches.